The geometry of bike handling: It’s all about the steering

Over the last 200 years, the design of the bicycle has evolved in a variety of ways to meet the needs of a diverse range of riding disciplines. Now there are dozens of specialised bikes on the market and the distinctions that separate them are often related to the way that they perform and behave, which in turn, is dictated by the geometry of the frame and fork.

Head angle, fork rake, trail, and wheel flop: these are just some of the parameters that influence the steering and handling of a bike, however in the absence of any context, they often appear as a set of meaningless numbers in a frame geometry chart. This is something that we started to decipher with our original article on the topic in 2011, and now, Matt Wikstrom revisits it to delve even deeper into the parameters that influence the handling of a bike.

When it comes to buying a new bike, most riders understand the importance of finding the right size frame. This is something that we have discussed previously in our article on sizing up a road bike where we identified those aspects of frame geometry that are important to fit.

There are other aspects to frame geometry, such as the head tube angle or the amount of trail, which have no bearing on the fit of the bike, yet they can still have an effect on the rider. That’s because they influence the way that the bike handles, be it on the ground, in a corner, up a hill, or even in the air. It’s an issue that is important to the safety of a rider as well as their confidence, and in some instances, it can assist their performance, too. If nothing else, when a bike is easy to control, it will elevate the rider’s enjoyment of the sport.

Two centuries of experience has taught us a lot about the way that a bike behaves and how to make it easier to control. Academics have dissected the dynamics of motion and defined the minimum requirements for a self-stable bike while the bike industry has experimented with various designs to define what works and what doesn’t work for every riding discipline.

While our understanding is not yet complete, all of this work has identified a core set of parameters that dictate the behaviour of a bike. There appears to be a hierarchy of sorts, because some parameters have a bigger impact than others, but in every instance, these effects can be traced back to one single mechanism, and that is the steering of the bike.

The geometry of any bike comprises a variety of measures and angles, but it is those that are important to steering that dictate how it will handle.

Steering to remain upright

Take a moment to think about what it was like learning to ride a bike: the thing had this horrible tendency to swerve, and quelling it seemed to be the secret to remaining upright. However, any effort that was made to stop the handlebars from turning ultimately proved futile because the bike would always capsize. The trick was in learning how to steer into, not away, from every fall, because that is what makes a bike remain upright.

It’s a counter-intuitive notion, to say the least, however the strategy works because it directs the wheels back under the falling mass. As long as the bike is moving quickly enough to “catch” the falling mass, the bike will rise every time. It’s a process that is akin to balancing a broomstick on your hand: as the stick starts to fall, the hand must be moved in the same direction to stop it from toppling over.

There are times, though, when it is necessary to cause a bike to fall. While this might sound like a terrifying prospect, there is no way to corner without it. That is why a bike, and the rider, always lean into a corner, because they are in the process of falling. Once again, it is the steering of the bike that is critical to the rider’s ability to control the fall and right the bike as they exit the corner.

Of course, steering is also important for controlling the direction of a bike, but it’s largely an auxiliary function until the rider has it under control. This can be seen in the way that a bike will swerve erratically at low speeds, because the rider cannot control the direction of the bike while they are struggling to keep it upright. Indeed, any time that a rider is having trouble controlling the direction of a bike, it is a sure sign that they are trying to stop it from falling over.

Steering with the hands versus the body

In broad terms, there are two aspects to steering a bike, the first of which involves the use of the handlebars. This is something that every rider intuitively understands, and with a little practice, steering with the handlebars becomes a subconscious process for both guiding the bike and staying upright.

The second involves steering from the saddle. Anybody that has ever guided a bike with the saddle while walking behind it will understand this notion: when a lean is initiated from the saddle, it prompts the handlebars to turn in the same direction. The same thing happens when riding no-handed: a bit of body English is all that is required to control the direction of the bike.

At low speeds, steering with the handlebars is critical for controlling the bike; at high speeds, steering from the saddle is more important, though the two are by no means mutually exclusive. Employing the two steering strategies seems to be an instinctive process, though it’s clear that practice will improve a rider’s skill at both.

If the control of a bike depended entirely on steering with the handlebars, then this wouldn’t be possible.

The geometry of the steering response

Given the importance of steering, it shouldn’t be surprising that the geometry of the front end of a bike has the biggest impact on its handling. This all starts with the head tube angle, but as will be explained below, this is subject to the rake of the fork, which dictates the amount of trail and wheel flop. Stem length, handlebar width and tyre width also have a role to play by modifying the response of the front wheel to input from the rider, as does the gyroscopic effect of the wheel itself.

There are other parameters related to the rest of the bike that also influence the way that it handles. These include the height of the bottom bracket, the length of the wheelbase, and distribution of the rider’s weight. All will be addressed in the sections that follow, though it is worth stressing that while each of these parameters can be dissected for the purposes of discussion, none of them act in isolation.

Head tube angle

Of all the factors that influence the handling of a bike, head tube angle is perhaps the easiest to understand. In simple terms, head tube angle dictates how much effort is required to turn the front wheel. As head angle increases, the front wheel becomes easier to turn. It is also possible to make a sharper turn, and thus, the manoeuvrability of the bike generally increases with the angle of the head tube.

At low speeds, a steep head angle can help a rider maintain their balance. Steering the bike with the handlebars is generally very easy, though there is a risk of oversteer, so a light touch may be needed. At higher speeds, quick and light steering becomes a liability as the bike will be prone to sudden changes in direction to the point where it may become uncontrollable.

This is when a slacker head tube angle is often a better choice: steering with the handlebars may be more difficult at low speeds, but the extra stability associated with relatively insensitive steering will moderate the behaviour of the bike at high speeds.

The difference in head tube angle associated with each of these extremes is quite small: for example, a purpose-built criterium race bike that is designed for cutting through tight bends and dodging traffic may have a head angle of 74°. In contrast, a downhill MTB that is able to contend with high speed descents on tricky terrain might have a head tube angle of 64°.

While there is more to the steering of a bike than simply the head tube angle, in general terms, it sets the tone for the handling of the bike. The next thing to consider is the influence of the fork, and more specifically, the amount of trail it creates.

Trail

When the contact patch for a wheel trails behind the steering axis of a fork, it will tend to align with the direction of the bike (like the wheels of a shopping trolley or an office chair). While this caster effect is not strictly necessary to create a self-stable bike, it becomes increasingly important at higher speeds by helping the front wheel remain centred without any input from the rider. If there is too much trail, though, then the bike will be difficult to steer.

Fork rake/offset (left) is one of the parameters that determines the amount of trail (right) for a bike.

The amount of trail that is present for a bike depends upon three factors: head angle, the amount of fork rake (or offset), and wheel size. When the size of the wheel and the amount of fork rake is kept constant, then the amount of trail increases as the head angle gets slacker (Figure 1). Under these conditions, the front wheel will tend to remain centred, accounting for much of the stabilising effect of a slack head angle.

Decreasing the amount of fork rake will also have the same effect (Figure 1), which may seem counter-intuitive, since it moves the front wheel closer to the bike. However, the position of the front wheel with respect to the rest of the bike is not important for this effect; what is important is where it is located in relation to the steering axis of the head tube.

Thus, for any given head tube angle, decreasing amounts of fork rake will move the contact patch for the front wheel away from the steering axis, where it will be more inclined to self-centre due to the caster effect. This is why the fork of a stayer bike is reversed, because the negative rake adds considerably to the amount of trail, and thus, the stability of the bike at high speeds.

Figure 1: The amount of trail for any bike depends upon the head tube angle and the amount of fork rake. Trail values are shown for a road bike with a 25mm front tyre.

Increasing the amount of fork rake will have the opposite effect: as the amount of trail gets smaller, the front wheel is less likely to follow the direction of the bike. With a large amount of fork rake, it is possible to eliminate trail altogether (or even create negative trail, where the contact patch sits ahead of the steering axis), however the steering will be very light, tending towards uncontrollable, even at low speeds.

For road bikes, it is generally accepted that 55-60mm of trail is ideal, providing a good balance of manoeuvrability and stability. Larger amounts of trail are typically used for gravel bikes and MTB, while less trail is often employed for loaded touring bikes (the weight of front panniers makes the front wheel more difficult to turn, hence the need for a quicker steering response).

Wheel flop

Wheel flop is similar to trail in that it is determined by the combination of head tube angle and fork rake, however it is concerned with how the position of the front axle changes as the handlebars are turned. In almost all instances, the height of the front axle is lowered when this happens, and this is accentuated by a slack head angle and/or less fork rake (Figure 2).

For the rider, the effect of wheel flop is most obvious at low speeds, when it will help the front wheel make a turn. A bike with a large amount of flop (e.g. 20mm) will be more willing to make a sharp turn than one with a low amount of flop (e.g. 10mm); thus, some wheel flop improves the manoeuvrability of a bike while too much will produce a bike that will wander off line with a light touch of the bars.

Figure 2: Wheel flop provides a measures of how much the height of the front wheel axle changes when the handlebars are turned. This measure increases as the amount of fork rake decreases as shown in the plot above for a 700C wheel fitted with a 25mm tyre.

One way to experience this effect is to hold a bike off the ground: when the back wheel is higher than the front, the bars will be reluctant to turn because there is very little flop; as the front wheel is lifted higher, though, less effort is required to turn the bars, and there will come a point when the front wheel refuses to remain centred.

In practical terms, wheel flop counteracts trail, however the two cannot be divorced. As one increases, so does the other, and it is not possible to design a bike which offers a large amount of trail with very little wheel flop, or vice versa.

Nevertheless, wheel flop can be used to distinguish the behaviour of different head angle/fork rake combinations that share the same amount of trail. For example, 71°/55mm, 72°/49mm, 73°/43mm, and 74°/37mm all produce 59mm of trail when paired with a 700C x 25mm tyre, yet the amount of flop differs, steadily decreasing from 18mm to 16mm. It’s a nuance, to be sure, but it demonstrates why the steering responses for these head angle/rake combinations will not be identical.

Gyroscopic effect

A lot of riders will be familiar with the gyroscopic effect associated with a spinning wheel. When held between the hands, these forces can be felt when trying to tilt and turn the wheel. On the bike, this effect will cause the front wheel to turn into any lean, and therefore, it will help right the bike. In the past, it was generally believed that this effect was critical for keeping a bike upright, however, just like trail, the gyroscopic effect of the wheels is not necessary for a self-stable bike.

A change in the weight of the wheels will influence the gyroscopic forces that help stabilise the bike.

Gyroscopic forces can help stabilise a bike, though, especially at high speeds, by keeping it upright and on line. With that said, this extra stability is not always desirable. For example, the reduction in gyroscopic forces that comes with an upgrade to lighter wheels is normally viewed as a benefit, and most riders will revel in the extra agility of the bike rather the ruing the loss of stability.

Stem length

The length of the stem is a little like fork rake in that it can modify the steering response of the bike, however it operates on a smaller scale. This is something that we have discussed in detail in our earlier article on how stem length influences the handling of a bike, so only the major points will be mentioned briefly here.

To start with, the size of the steering arc for the handlebars depends upon the length of the stem. When this arc is small, the handlebars are more sensitive to input from the rider (i.e. hand and arm movement), and thus, less effort is required to change the direction of the bike. A longer stem creates a larger steering arc that is less sensitive to hand and arm movement, and this slows down the steering.

A longer stem also moves more of the rider’s weight over the front wheel. This is especially true when comparing frames with different amounts of reach: while a short frame and long stem will provide the same overall reach as a long frame and short stem, a larger fraction of the rider’s weight will end up over the front wheel with a long stem. As a result, the response of the front wheel will be slowed further.

A short stem, by contrast, will shift the weight of the rider back over the frame, unweighting the front wheel so that the steering will be a little lighter. As already mentioned, this won’t to do much to overhaul the steering if the bike has a slack head angle and/or lots of trail, but it can improve manoeuvrability, which explains (at least in part) the current fervour for short stems on MTB (where the amount of frame reach has been growing in recent years). It also explains how a short stem can render a bike near-uncontrollable if it already has quick steering.

Handlebar width

Handlebar width can have the same kind of effect on the steering response of the bike as stem length. Wider bars spread the hands apart (especially when the hoods or drops are held) and this increases the size of the steering arc of the bike. Narrow bars make for a smaller arc, and therefore, quicker and more sensitive steering. This can improve the manoeuvrability of the bike, but beyond that, it can produce twitchiness, especially when the bars are paired with a short stem.

In the case of drop bars, a longer reach and/or brake hoods will also add to the size of the steering arc. Once again, this will slow the steering response, so it is worth considering the combination of the two when selecting the components for a new bike.

The width of the handlebars has another effect on the handling of a bike by stabilising the rider’s body. Anybody that has tried to ride out of the saddle with their hands next to the stem will understand this immediately: more strength is required to stabilise the body, and the bike, too, becomes difficult to control. Thus, wider handlebars will help stabilise a rider, especially if they have wide shoulders, even when they are seated in the saddle.

Finally, there is the effect that handlebar width has on steering torque. Wide bars provide extra leverage for turning the front wheel, which becomes important when wide tyres are used for off-road riding. In this regard, the value of wide handlebars to MTB is already well known, however the promise of extra torque explains how wider bars and/or flared handlebar drops might be helpful for gravel bikes and loaded bike-packing/touring rigs.

Tyre width

The width of the front tyre can be used to adjust the trail of any bike. That’s because the diameter of a wheel generally increases with the width of the tyre, which in turn, will increase the amount of trail. For example, changing from a 700C x 23mm tyre to 28mm tyre will add 2mm of trail while a 40mm tyre will increase it by 6mm (Figure 3). In most cases, a small increase in trail is likely to go unnoticed, however larger changes, like that seen for a 40mm tyre, will tend to dull the steering response.

Fitting a wider tyre will normally increase the diameter of a wheel, which in turn, will have an effect on the steering of the bike.

The amount of wheel flop will also increase with the width of the tyre. Continuing with the example cited above, a 28mm tyre will increase wheel flop by ~1mm while a 40mm tyre will add ~2mm of wheel flop (Figure 5). Once again, a small change is unlikely to be noticed, but in the case of a 40mm tyre, the combined effect on trail and flop is equivalent to reducing the head angle of the bike by almost 1°.

There is another effect associated with tyre width that also influences the steering of a bike, which is known as pneumatic trail. This is where forces associated with the contact patch of the tyre act in opposition to any steering input. Pneumatic trail generally increases with the width of a tyre and will work, firstly, to keep the wheel aligned with the direction of the bike, and secondly, to return it to centre after a turn is initiated. Unfortunately, there is no handy metric for this effect, so the impact on the steering of a bike is much harder to anticipate.

Figure 3: The amount of trail and wheel flop for a bike increases with the size of the front tyre, though the change in wheel flop is much smaller than the effect on trail.

In the case of a road bike, where the contact patch is relatively small, this effect is small and can often be ignored. Wide tyres (e.g. 40mm) are a different matter though, which is why more steering torque may be needed to compensate for a reduction in manoeuvrability.

All of these effects help explain why a significantly larger tyre (e.g. for gravel riding) will always slow the steering of a bike and render it more stable at high speeds. The steering won’t feel as light or precise at low speeds, either, making for quite a contrast in behaviour unless a smaller wheel diameter (e.g. 650B) is employed to offset some of these effects.

In terms of preserving the geometry of the bike, this is a sound strategy, as can be seen in the following example: for a bike with a 71° head angle and 55mm fork rake, the amount of trail and wheel flop associated with a 700C x 25mm tyre is 59mm and 18mm, respectively. Those numbers increase significantly to 68mm and 21mm when a 50mm tyre is fitted to the same wheel; by contrast, a 650B wheel with a 2.1inch-wide tyre will yield 62mm of trail and 19mm of flop. Some slowing of the steering can still be expected, however much of that will be due to pneumatic trail.

Swapping between 700C and 650B wheel sizes allows wider tyres to be used with only minor alterations to the amount of trail and wheel flop for the bike.

Bottom bracket drop

At face value, the bottom bracket seems irrelevant to the handling of a bike, and in one sense, this is true, because it has no influence on the front-end steering of a bike. It does, however, have a role to play in determining the position of the rider, and more specifically, the location of their centre of gravity.

Consider for a moment how a road bike behaves when descending at high speed with the hands on the hoods versus the drops. The latter is always more stable because the rider has lowered their centre of gravity. The position of the bottom bracket exerts its influence in the same way: a relatively low bottom bracket places the rider closer to the ground, and that generally acts to stabilise their position on the bike.

The position of the bottom bracket can be described in two ways: first, there is the distance to the ground; and second, there is the position of the shell in relation to the centre of the wheels. The first measure is most important for understanding the amount of clearance a bike has to offer, while the second provides a better idea of the effect on a rider’s centre of gravity.

The height of the bottom bracket can be measured in relation to the ground as well as the wheel axles. The former will depend on wheel and tyre size while the latter can be directly compared for different bikes.

If a line is drawn through both wheel axles to form a horizon, then the bottom bracket is normally located below it (hence the term bottom bracket drop). Just how far depends upon the design of the frame. Road frames normally offer 65-75mm of bottom bracket drop, however it can be a little more in some circumstances (e.g. touring bikes can have a drop of 80mm or more). MTB frames, by contrast, typically opt for less bottom bracket drop because extra ground clearance is required to avoid pedal and chainring strikes on rocks and other trail obstacles.

Given the fact that every bike requires an amount of ground clearance, there is a limit to how far the bottom bracket can be lowered to stabilise a rider. Nevertheless, for those riders that have an upright position on the bike, there is the promise of extra stability at high speeds when steering with the body, though it’s likely to be a matter of nuance. What is arguably more important is how the rider’s weight is distributed between the front and rear wheels, which is where the wheelbase and front- and rear-centre measurements of a bike become important.

Wheelbase and weight distribution

The wheelbase of a bike describes how far the front wheel is separated from the rear wheel, and is measured from the centre of one axle to the other. This distance is largely dictated by the length of the frame, so as the size of a frame increases, so does the wheelbase.

When the wheels are close together, the bike has a smaller turning circle, and therefore, it is much easier to guide it through tight turns. Increasing the length of the wheelbase generally stabilises a bike, which is why downhill MTB have longer wheelbases than XC race bikes, but this comes at the expense of manoeuvrability.

While the overall length of the wheelbase sets the tone for some of the stability and manoeuvrability of a bike, it will be influenced by the distribution of the rider’s weight, which can seesaw from the front to the back wheel, depending on where the bottom bracket is located. This is where the front- and rear-centre distances become important because they provide a measure of how far each wheel is located from the bottom bracket.

The front-centre measurement is normally larger than the rear-centre (60/40 is a common split) since the front wheel requires extra room for turning. The front-centre distance always depends upon the reach of the frame, however, a slack head tube angle and/or more fork rake will push the front wheel further away from the bottom bracket. In contrast, the rear-centre depends upon the length of the chainstays.

The difference between the front- and rear-centre distances means that a larger proportion of weight generally ends up over the rear wheel, though the exact amount depends upon the rider’s position. An aggressive race position, or the use of aero extensions, will transfer more weight onto the front wheel to slow the steering. In the right measure, this can improve the stability of the bike — indeed, some have suggested this is important for preventing speed wobbles — but an excess will make minor steering corrections more difficult, robbing the rider of fine control.

Less weight on the front wheel, by contrast, will make for lighter steering, increasing the risk of oversteer. At the same time, there will be a reduction in tyre grip, which can be unnerving, even dangerous, in slippery conditions. The net result may be a bike that behaves erratically, especially at high speeds, and requires more effort (and concentration) to control.

The effect of weight distribution is not limited to the front end of the bike, because it also affects steering from the saddle. Extra weight will accentuate any lean that the body makes, and therefore, the tip and fall of the bike. This effect is most obvious when riding no-handed, but it can also be produced by adding weight to the rear wheel in the form of loaded panniers. In both instances, the bike will be more prone to swerving, especially at low speeds, though the threat of veering off course will persist at higher speeds.

A rider normally has a lot of freedom to adjust their position on a bike and this will have an impact on their weight distribution as well as the handling of the bike. Photo: Antton Miettenen.

Clearly, finding a good balance in weight distribution will moderate both steering mechanisms so that the rider can enjoy the benefits of each. There is no hard-and-fast rule for what that might be, though; some have suggested that a 45/55 (front/rear) split is ideal for a road bike, but this will be subject to the intrinsic steering response of the bike. For example, if the bike has a relatively slack head angle and a large amount of trail, moving some weight onto the rear wheel can improve the steering response of the bike.

Stem length and saddle position can be used to redistribute the weight of a rider, however the range of adjustment is limited to just a couple of centimetres, and even then, not all riders will have the freedom to do this. This is especially true for road cyclists where re-positioning the hips can ruin the effectiveness of their pedal stroke while a change in reach may compromise comfort. Nevertheless, small refinements in both may affect the handling of the bike.

Such restrictions disappear completely when a frame is designed from the ground up. Indeed, optimising weight distribution is often an important goal for a custom-made frame. In many cases, this is not the kind of thing that will overhaul the behaviour of the bike, but it promises to add a level of refinement to the finished product.

The influence of the rider

The influence of the rider extends beyond the distribution of their weight. After all, they are a highly active and responsive element in this system. Their legs are in constant motion and they will makes all sorts of voluntary and involuntary actions in response to both the behaviour of the bike and the terrain.

One good example of this concerns their proficiency at steering. Novice cyclists tend to make exaggerated, even erratic, motions when steering a bike with the handlebars as well as their bodies. Practiced cyclists, by contrast, are more precise at both activities, and this makes for a marked difference in the amount of input for the bike.

If a bike has highly responsive steering (e.g. steep head tube angle and low trail), then the extra input from a novice will be amplified to the point where it may become unstable and difficult to control. A practiced rider won’t have to contend with the same temperament because they are able to control the bike with a light touch.

A rider is more than a passive weight on the bike, which is why their skill and responses have a big impact on the behaviour of the bike.

Riding style also has an influence. This can mean all sorts of things, depending on the discipline, but in simple terms, it refers to the way that the bike is used. Some examples include racing at high speeds versus leisurely social rides; commuting on quiet paths versus dodging peak-hour traffic; braking early versus late; or hopping over obstacles versus carrying the bike. While a rider has some conscious control over these aspects, their riding style will shape their habits and instincts, which in turn, will influence the amount of input for the bike.

Yet another variable relates to the rider’s perception. Only a little is known about the way that a rider perceives the behaviour of a bike, however there is likely to be a sensory threshold that varies from one individual to the next. If an individual is more sensitive to the tip or swerve of a bike, then they may be prone to over-correction, while an amount of insensitivity could have the opposite effect. Once again, this will make for a difference in input from the rider that can be amplified or diminished, depending upon the geometry of the bike.

A rider’s behaviour can change as they become accustomed to the handling of a bike, though. Human beings are remarkably adaptable, and given enough practice, many are able to learn how to control all sorts of bikes (including some very odd contraptions).

In this regard, the minor variations in steering and handling that now distinguish many of the bikes created for any discipline (road cycling is a good example, see below) can be considered largely inconsequential because most riders are able to quickly adapt to them. However, some will still be better suited to an individual than others, if only on the basis of personal preference.

Bringing everything together

Head tube angle, fork rake, trail, wheel flop, gyroscopic effect, stem length, handlebar width, tyre width, bottom bracket drop, wheelbase, weight distribution, and the rider: having discussed all of these parameters, the next issue to understand is how they all come together to define the handling of the bike.

Unfortunately, there is no simple answer to this question. Our understanding of this issue has not progressed to the point where we have a handy formula, or even a complicated algorithm, that can crunch the numbers to predict how a bike will handle.

With that said, some equations have been derived for calculating the angle and rate of lean and steer that can be used to predict weave and capsize with respect to speed, but this is only relevant to the self-stability of a bike. The rider has no place in these equations other than as a passive mass, which is hardly the case in the real world. Moreover, there is no indication yet that a self-stable bike is actually easier to ride (or safer) than a bike that is less stable.

At this point, we know enough about steering to predict the behaviour of a riderless bike, but that is a long way from modelling how a bike will behave for a rider.

Academics are tackling the challenge of modelling the behaviour of a cyclist with the hope that more robust models can be developed to predict the behaviour of a piloted bike. In the long-term, there is the hope that these models can be used to design safer bikes, but it may prove useful for exploring new geometries, too.

There is another approach, though, and it is one that the bike industry has embraced from the very beginning: empirical testing. The earliest framebuilders simply started experimenting with different head tube angles and fork rakes, and from there, our understanding slowly grew on the basis of rider feedback.

While this sort of information is enormously practical, it doesn’t provide much insight on the mechanisms at play. All that it can provide is a group of associations stemming from cause-and-effect. Put another way, empirical testing has provided us with a pretty good understanding of what works and what doesn’t, but we can’t really say why (though there are often plenty of opinions).

Much of our wisdom on steering is therefore contained in the geometry that has emerged over the last two hundred years, and anybody hoping to learn more need only interrogate it to gain some insight. For example, a simple comparison of the geometry associated with different riding disciplines (Table 1) provides a feel for just how much each steering parameter can be varied to alter the handling of a bike.

Table 1: Comparison of frame/fork geometry for bikes designed for different riding disciplines from a single manufacturer (Specialized).

A wider survey is needed to understand that there is a limit to this kind of adjustment before a bike becomes difficult to control, which isn’t really surprising given the finite nature of the physics involved. So while it is possible to build a frame with a wide range of head tube angles, the most effective values are clustered around 70°. Likewise, the other steering parameters, to the point where some reasonably robust templates can be derived from existing geometries.

This is most obvious when comparing geometries for a single discipline, such as road cycling (Table 2). Then, the distinctions that separate the various brands and models of bikes on the market are often measured in millimetres and fractions of a degree, especially for similarly-sized frames. With that said, not all disciplines have been defined to same extent. Gravel riding, for example, is still evolving under the influence of geometries from cyclocross, touring, and MTB.

Table 2: Comparison of frame/fork geometry for a variety of road bikes with disc brakes. There is more variation in the stack and reach of these bikes than there is in the steering geometry.

When minor variations in the steering parameters are put to the test, then the result is typically a matter of nuance. However, when these kind of nuances are crafted to meet the needs of the individual, then they can make for an amount of refinement. Indeed, custom framebuilders have made an art out of refining frame geometry to suit the individual, so it is interesting that few, if any, adopt a formulaic approach to frame design.

Instead, the geometry of the frame is developed on a case-by-case basis in response to customer’s fit, position, riding style, experience, and preferences, where a variety of strategies may be used to refine the final product. There is much more art than science in this process, which accounts for the variety of styles and ideologies in the bespoke market.

Final thoughts

The geometry of a bike is a recipe of sorts, and with some understanding of the ingredients, it is possible to anticipate what the final dish will taste like. However, the flavours of the meal are still likely to surprise, more or less, depending on the palate.

The bicycle industry has come a long way over the last two hundred years, and while we can expect more experimentation and refinement, a studious approach to empirical testing has taught us a lot about the way the geometry of the frame and fork can influence the steering and handling of the bike. Some parameters, such as head tube angle and trail, may have more impact on the steering of the bike than others, but the final result is always the product of every ingredient in the recipe.

Thus, in many ways, the geometry of a bike is much like its ride quality in the sense that it is something that can be experienced and assessed with an amount of objectivity, however it remains a little mysterious. For those that are intrigued by the nuances of bike handling, the only way to learn more is to educate the palate with a bit of empirical testing. If nothing else, it will help riders understand the range of experiences that can be had with relatively minor changes in steering geometry.

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